Sometimes you don't have the time or opportunity for lengthy hand-sewn repairs. In such cases, iron-on patches can be perfect for a quick fix for a hole or a stain.
Although they are designed to be ironed on as they are, I recommend sewing them on as well if possible, as it makes them much more durable. But you can iron them on quickly first if needed, and then sew them on at a later time.
It's easiest to do on a machine since iron-on patches are stiff, and it might be beneficial to use a darning foot if you have one, and then remove the feed dogs so you can move the fabric/garment in any direction you want. Feel free to practice on a fabric scrap with a ballpoint pen drawing of a heart or star or something. The stitches don't need to be perfectly even; the most important thing is that the patch is securely attached :)
Iron-on patches are a bit stiff, so if there's a very large worn area with a small hole, I would recommend attaching a reinforcement patch to the inside of the garment first, either with machine stitching (a bit like quilt style with stripes or checks, perhaps with thread matching the garment?) or by using a fabric patch and Vliesofix, and ironing it on the inside, and then covering the hole or stain with the iron-on patch. Of course, you can also use iron-on patches to decorate clothes that have no damage or wear!
Reinforcing the inside of worn areas is beneficial so that you don't get a new hole exactly at the edge of the iron-on patch, where there will be extra stress where the stiff meets the thin/worn fabric.